1. Field of the Invention
The present invention relates to a mold and a method of manufacturing the same. More specifically, the invention relates to a mold suitable for manufacturing precision parts such as optical elements formed of glass, as for example, a lens and a prism, by press-molding glass materials, and to a method of manufacturing such a mold. Particularly, the invention relates to a mold having a carbon film formed thereon, provided with a mirror surface having minimum defects, free from film peeling and cracks, and also relates to a method of manufacturing such a mold.
2. Description of the Related Art
A technique for press-molding materials, such as glass materials, is known as a method of manufacturing precision parts, typically, optical elements such as lenses, which do not require a polishing process, or require only an extremely short polishing process.
Such a technique eliminates complicated processes, such as a polishing process required for conventional manufacturing, or it significantly shortens such processes, thus enabling simple production of lenses at low cost. Hence, today, such a technique is coming into use for manufacturing precision parts, typically, optical elements, such as lenses, prisms and other types of glass articles.
A mold used for press-molding glass optical elements requires good characteristics, such as hardness, heat resistance, releasability and mirror-surface processing characteristics. For a mold of this type, many proposals have been made using metals, ceramics, and materials coated with such metals and ceramics. Examples of such molds are: 13Cr martensite steel in Japanese Patent Laid-Open No. 49-51112, SiC and Si.sub.3 N.sub.4 in Japanese Patent Laid-Open No. 52-45613, a material obtained by coating a superhard alloy coated with a noble metal in Japanese Patent Laid-Open No. 60-246230, a thin diamond film and a diamond like carbon film in Japanese Patent Laid-Open Nos. 61-183134, 61-281030 and 1-301864, a hard carbon film in Japanese Patent Laid-Open No. 64-83529, a material coated with a carbon film in Japanese Patent Laid-Open No. 64-83529, and a material coated with a carbon film in Japanese Patent Publication No. 2-31012.
However, 13Cr martensite steel is likely to oxidize and Fe is diffused in glass at high temperatures, thus causing glass to be colored. Although SiC and Si.sub.3 N.sub.4 are generally considered unlikely to oxidize, they do oxidize at high temperatures and a SiO.sub.2 film is formed on the surface, causing glass to adhere to the mold. Moreover, the mold has extremely poor processing characteristics due to its high level of hardness. It is unlikely that a material coated with a noble metal allows glass to stick to the mold, but the surface of the mold is extremely soft so that it is easily scratched and deformed.
In contrast, a thin diamond film has a high level of hardness and superior heat stability. Nevertheless, since the thin film is polycrystalline, the surface of the film is extremely rough, and needs to be mirror-processed. Molds using a DLC (diamond like carbon) film, a-C:H (hydrogenated amorphous carbon) film and a hard carbon film exhibit good releasability between the molds and glass, thus avoiding having glass adhere to the molds. However, after molding is performed hundreds of times or more, the film might be partially peeled and a resultant molding might fail to perform sufficient molding.
This may result because of the following three reasons.
(1) All the foregoing films have an extremely large compressive stress, and film peeling and cracks occur due to stress relief along with rapid heating and cooling during the molding process. Similar symptoms can be seen due to a disparity of heat expansion efficiency between the matrix and the film and due to heat stress caused by a heat cycle. PA1 (2) There are some cases where the film cannot be entirely formed, or, even though it is formed, the thickness of the film is thin, depending on the surface condition of the matrix. For example, the matrixes obtained by using sintered bodies, such as WC-Co, SiC and Si.sub.3 N.sub.4, cannot avoid crystal grains missing and pores from being produced in the course of sintering, thus resulting in holes of a few .mu.m or greater forming on the polished molding surface. In the formation of a film on such a surface, the film cannot be formed on the portions with the above-mentioned holes, or even if it is formed, the film is extremely thin. Adhesion strength of such portions of the film and mechanical strength thereof are considerably lowered, and accordingly, film peeling and cracks are likely to occur starting from the portions with the holes. PA1 (3) An alloy is formed between a sintering aid in a sintered body, typified by Co in WC-Co, and the above-mentioned film, due to diffusion. Glass adheres to the portion having the alloy and the alloy reacts with components contained in glass, thus producing precipitates, resulting in a decrease in durability.
As described above, although the mold simply employs a carbon film, a thin diamond film, a DLC film, a a-CiH film, and a hard carbon film, there is still room for improvement in order to realize a mold which is suitable for molding optical elements and which is superior in all conditions, such as molding characteristics, durability and cost efficiency.
Japanese Patent Publication No. 2-31012 discloses that a film having a thickness of less than 50.ANG. becomes inconsistent, thus reducing the practical effects of forming the carbon film, while a film having a thickness of more than 5000.ANG. increases profile irregularities by pressure molding. In contrast, a film having a thickness within a range from 50 to 5000.ANG. does not present any problem. However, the carbon film in the embodiment of this patent publication is likely to peel in the molding process due to a slight adhesion to the base or due to a large compressive stress. This causes glass to adhere to the peeled portions, thereby spoiling the outer appearance of a resultant molding. Therefore, it is desirable to provide a practical mold having superior durability.